The invention relates to dense wavelength division multiplexing DWDM) systems which transmit multi-channel information and can efficiently expand the transmission.
Optical fiber communications systems are the backbone of communications networks and have been rapidly expanding in the past ten years. However, the data handling capacity of many existing systems is limited and inadequate to support many applications. Among known methods for increasing the data handling capacity of optical fibers are optical dense wavelength division multiplexing (DWDM), frequency division multiplexing (FDM), and time division multiplexing (TDM). In an FDM system, a particular sub-carrier frequency is assigned to each signal source, and a complete signal is constructed by combining each sub-carrier frequency. While this is a useful technique for transmitting multiplexed signals, its capacity is limited by the need for a high signal-to-noise ratio, requiring, higher power systems which in turn limits the number of data channels due to interference, cross talk and the nonlinearity of the optical waveguide media due to inadequate separation of the channels. In a TDM system, an electronic switch (multiplexing unit) picks up the signal on each input channel in order of channel by channel at the transmitting end. The multiplexed signal is transmitted through a medium (optical fiber) and distributed to related terminal equipment at the output of the system receiver. This is an efficient approach for transmitting the signal when the data rate is less than 2.5 Gb/s. Unfortunately it is a very expensive system when the data rate of transmitted signals is more than 3 Gb/s, due to the inherent inefficiency in electronic switching from channel to channel.
In a DWDM system, multiple optical signal channels are carried over a single optical fiber, each channel being assigned a particular optical wavelength. The information capacity carried by each channel is typically between 2.5 Gb/s and 10 Gb/s. It is an efficient and cost effective method for increasing the capacity of existing optical fiber communication systems. A bidirectional DWDM system operating independently of the signal data rate and format is commercially available from Osicom, the details of which are described in United States Letters patent application Ser. No. 09/004,984, which is hereby incorporated by reference which explicitly teaches a short distance DWDM optical transmission system without any amplification or regeneration. However, in the existing Osicom system, in accordance with the relevant ITU specifications, the channel spacing between adjacent channels that are be transmitted over a single optical fiber is 0.8 nano meters (xe2x80x9cnmxe2x80x9d) and is dictated by the need to avoid cross talk and other forms of data corruption between adjacent channels; such a relatively wide channel spacing permits the combined wavelength multiplexed optical signal to be spit into its constituent single wavelength components and permits those single wavelength components to be combined into the combined wavelength multiplexed signal using relatively simple and inexpensive filter-based optical multiplexers and demultiplexers without any mechanism for re-shaping or otherwise processing the individual data pulses in accordance with any predetermined data rate and format.
What is needed is an optical communications system which can transmit data bidirectionally with a minimal spacing between channels (thus maximizing the number of transmission channels) at a minimal cost in terms of data loss, functionality and reliability, that is compatible with existing optical components, and that is preferably independent of the data rate and format.
In a first aspect, the present invention provides a bidirectional optical transmission system, for transmitting unidirectional signals over an optical waveguide, comprising a first transmitter for transmitting signals in a first direction, and a second transmitter for transmitting signals in the opposite direction, each transmitter comprising a plurality of wavelength converters, each converting a different one of the unidirectional signals into a modulated optical signal centered about a different respective optical wavelength, and a respective wavelength multiplexer coupled to the wavelength converters, for combining the individual signals into a combined signal; a first receiver for receiving signals from the first transmitter and a second receiver for-receiving signals from the second transmitter, each receiver comprising a wavelength demultiplexer for separating the combined signal into received optical signals, and optical receivers, each responsive to a different one of the optical signals and converting it back into its original form; and a first directional guide means for coupling the first transmitter and the second receiver to one end of the waveguide, and a second directional guide means for optically coupling the second transmitter and the first receiver to the other end of the waveguide, each optical directional guide means comprising an input port for receiving the combined signal from a wavelength multiplexer before the combined signal has been is transmitted over the waveguide, an output port for transmitting the other combined signal to the other wavelength demultiplexer after the other combined signal has been transmitted over said waveguide, and a bidirectional port for transmitting the combined signal to the optical wave guide and for receiving the other combined signal from the optical waive guide; wherein a first plurality of information-bearing signals are modulated, combined and transmitted from the first transmitter to the first receiver in a first direction and a second plurality of unidirectional signals are modulated, combined and transmitted from the second transmitter to the second receiver in a second direction, concurrently and bidirectionally over the same said optical waveguide, adjacent optical wavelengths of either combined signal have a predetermined first channel spacing providing a predetermined minimal amount of cross talk between two adjacent channels regardless of the direction of transmission of the individual optical signals, whereby each said combined optical signal is compatible with wavelength multiplexers and wavelength demultiplexers that are designed for said predetermined first channel spacing between adjacent channels; adjacent optical wavelengths transmitted over the optical waveguide have a predetermined second channel spacing substantially less than said first predetermined channel spacing, whereby the maximum transmission capacity of said system with only one said waveguide is greater than that possible with a waveguide having said predetermined first channel spacing between adjacent channels; and adjacent channels of said optical waveguide are not used for transmission in a same said direction.
In other aspects, the present invention provides at least some of the first individual wavelengths of the first combined signal may be interleaved with the second individual wavelengths of said second combined signal; substantially all of the first individual wavelengths of said first combined signal may be interleaved with the second individual wavelengths of said second combined signal; the optical directional guide means may be optical circulators that guide essentially all of each said combined signal to a respective intended destination port; the optical directional guide means may be polarizers circulators that guide essentially all of each said combined signal to a respective intended destination port; each said predetermined spacing is measured from the nominal center of one channel to the nominal center of an adjacent channel; the first predetermined spacing is twice said second predetermined spacing; the first predetermined spacing is 0.8 nm and said second predetermined spacing is 0.4; the plurality of wavelength converters further comprises a photo detector for directly converting an optical input signal into a respective electronic signal; an electronic signal amplifier, for amplifying said resulting electronic signal; a laser; and a laser driver for directly modulating the output of the laser according to said resulting electronic signal; wherein each said laser produces a modulated optical signal at a different said center wavelength; and the first and second plurality of individual modulated signals are modulated, combined and transmitted, without regard to data rate or format.
These and other features and advantages of this invention will become further apparent from the detailed description and accompanying figures that follow. In the figures and description, numerals indicate the various features of the invention, like numerals referring to like features throughout both the drawings and the description.